Under a natural environment, wood materials made from most of tree species or natural fibers are susceptible to the attack of fungi, molds or mildews, insects, and other biodegrading agents. Especially in a warm and wet condition, an untreated wood material can readily deteriorate. For example, a wood material is subject to decay by fungi when it has a moisture content greater than 30% and stays at an environmental temperature of between about 50° F. to 90° F.
By being treated with certain chemicals, wood preservation products can effectively prevent the attack by the aforementioned biodegrading agents. In general, most of the wood preservation products are durable. They can directly be used without any paint protection.
However, the wood preservation products still absorb water and moisture during service. The absorbed water in the vessel elements, fiber tracheid, or other microstructural cavities of the wood preservation products moves freely by capillary action in a natural atmosphere, thus resulting in the swelling and shrinking of wood. Along with the season and climate changes in a natural environment, the swelling and shrinking stress in wood may make the wood preservation products: 1) to have dimensional and shape changes, 2) to develop surface cracks and splits, 3) to lose the required mechanical properties, and 4) even to be rotted down by fungi.
Another disadvantage of the most wood preservation products is the leaching of chemical preservative ingredients from wood during service. For example, Hingston and coworkers (2000) have reported that 25% of the copper element was leached out from CCA-treated pine in the first six months. The maximum leaching of copper was as high as 52% within 85 months. During service, the leaching of a preservative from the wood preservation products will not stop until the preservative reaches to a constant retention level at which few unfixed preservative compounds or complexes exist in wood. In addition, the aforementioned cracking defect may accelerate the leaching of preservatives from wood.
Most of the wood preservatives are very toxic to human beings and animals, while some are even carcinogenic. The leaching of unfixed preservatives from wood during service may be a severe threat to the natural environment and human health. Moreover, the toxic chemical leachates may corrode soil, contaminate water, and influence Earth's natural cycles. Accordingly, these leachates are a potential danger to agricultural crops, wildlife, livestock, human beings, and the natural environment.
In order to overcome these defects, some wood preservation products are coated with conventional wood paints such as lacquer, latex, acrylic, and the like (Stilwell and Musante 2004). However, most of them cannot maintain good water and weathering resistances because they are either easily peeled off from wood during service or cannot stop the wood from cracking.
Sometimes a wood preservation product can be coated with epoxy or polyurethane oil-based paints to meet a requirement for water and leaching resistances. As is well known, these oil-based coatings usually take several days to be completely cured before they can be handled or touchable. Hence, this approach would need more time and storage space to dry these oil coatings during production.